Referring to FIG. 1 showing a schematic diagram of a conventional phase-locked loop (PLL), the PLL comprises a phase frequency detector 10, a charge pump 20, a loop filter 30, a voltage-controlled oscillator (VCO) 40, and a frequency divider 45. A reference signal having a reference frequency Fref, generated by a reference oscillator (not shown), for example, and a frequency divided signal from the frequency divider 45, are concurrently inputted into the phase frequency detector 10. The phase frequency detector 10 detects differences of the phase and the frequency between the reference signal and the frequency divided signal, and then outputs a phase difference signal to the charge pump 20. The charge pump 20 then generates an output current associated with the phase difference signal, according to the amplitude of the phase difference signal, to the loop filter 30. After smoothing the output current, the loop filter 30 converts the output current into a voltage control signal to the VCO 40. According to the control signal, the VCO 40 generates a voltage-controlled signal with a voltage-controlled frequency Fvco. The frequency divider 45 receives the voltage-controlled signal Fvco, and divides the same by N to generate a divided frequency signal, where Fvco=N*Fref.
The VCO 40 typically includes two types, namely, an LC oscillator and a ring oscillator. An LC oscillator has an advantage of having a low phase noise, but disadvantages of having a large layout area and a narrow tuning range. In contrast, a ring oscillator has advantages of having a small layout area and a wide tuning range, but a disadvantage of having a high phase noise. For implementing an integrated circuit, a VCO in a PLL is commonly realized by a ring oscillator. If a VCO in a PLL is to be realized by using an LC oscillator, an additional pin is needed for connecting with inductance components.
FIG. 2 shows a circuit diagram of a ring oscillator comprising multi-stage delay cells. A phase shift between an input terminal and an output terminal of each stage delay cell is controlled by adjusting the voltage controlling signal to generate a desired frequency. Further, an output terminal of a preceding stage delay cell is coupled to an input terminal of a subsequent stage delay cell.
From FIG. 2, delay cells 102, 104 and 106 are identical differential amplifiers, and receive a voltage controlling signal Vc to control the phase shift of the delay cells 102, 104 and 106. The positive output terminals Vo+ of the delay cells 102 and 104 are coupled to the positive input terminals Vin+ of next-stage delay cells, respectively; the negative output terminals Vo− of the delay cells 102 and 104 are coupled to the negative input terminals Vin− of next-stage delay cells, respectively. The positive output terminal Vo+ of the delay cell 106 is coupled to the negative input terminal Vin− of the delay cell 102; the negative output terminal Vo− of the delay cell 106 is coupled to the positive input terminal Vin+ of the delay cell 102.
The foregoing delay cells are not limited to differential amplifiers; a single-end circuit, such as an inverter chain consisted of inverters, may also serve as delay cells in the VCO, in which the phase shift of the delay cells may be controlled by the voltage controlling signal.
In general, in the VCO, a constant (Δf/ΔV) of a voltage-controlled frequency range over a controlling voltage is defined as Kvco. A voltage-controlled frequency of a conventional VCO is incapable of providing a constant Kvco. Kvco fluctuates according to changes in the controlling voltage. For circuit design, when Kvco fluctuates more seriously according to changes in the controlling voltage, the overall circuit design of the PLL gets more complicated. More particularly, a design of a preceding circuit unit coupled to the VCO, that is, the loop filter, gets extremely complicated. When Kvco fails to maintain at a constant value, the noise in the loop filter increases. Therefore, it is one of the objectives of the invention to provide a delay cell for use in a ring oscillator whereby a VCO is capable of providing a constant Kvco.